The smallest, coolest exoplanet known to host water is roughly the size of Neptune. Until now, astronomers had found water only on distant worlds that are about the size of Jupiter. The newfound water source — known as HAT-P-11b — is just a bit more than four times the diameter of Earth. And this distant planet’s temperature? It’s a very mild 605° Celsius (plus or minus 50°), or 1,121° Fahrenheit (plus or minus 90°). That’s hot enough not only to vaporize water but also to melt lead. So this would hardly be an inviting vacation destination. Jonathan Fraine is an astronomer at the University of Maryland in College Park. He and his colleagues discovered the distant planet’s water after a year and a half of observations with the Hubble, Spitzer and Kepler space telescopes. The researchers shared their news Sept. 25 in the journal Nature. Gases such as water vapor leave their mark in a planet’s atmosphere. They absorb particular colors (or wavelengths) of light. When HAT-P-11b comes between Earth and its star, the planet’s atmosphere filters out some of the starlight. The astronomers noticed the disappearance of some of that star’s infrared light each time the planet passed between it and Earth. The planet’s host star is an orange dwarf. It resides about 122 light-years away in the constellation Cygnus. The new analysis showed the planet has a relatively clear atmosphere, rich in hydrogen. All of that hydrogen is consistent with theories of planet formation. They hold that giant planets made from gas initially formed around a rocky or icy core. That core would quickly have attracted an atmosphere by pulling hydrogen out of the gaseous disk encircling an infant star. The new data “reveal the crystal-clear signature of water-vapour,” says Eliza M.R. Kempton. She’s an astronomer at Grinnell College in Iowa. “From the strength of the absorption,” she says, it appears “the planet’s atmosphere has a composition not dissimilar to those of the giant planets of our solar system.” It consists mostly of hydrogen, with trace amounts of heavier elements, she writes in a report published in the same issue of Nature. Those heavier elements include oxygen in the form of water vapor.